1. Field of the Invention
The present invention relates to an oil-free compressor arrangements and, more particularly, to a compressor arrangement including an oil-free screw compressor and a turbo-supercharger to produce a clean compressed air free of oil mist.
2. Description of the Prior Art
An oil-free type screw compressor includes a male rotor and a female rotor which is rotated, while maintaining a non-contact meshing relationship with each other, that is, a slight clearance is maintained between the male rotor and the female rotor at all times during rotation, with a casing forming a compression space in which said male rotor and said female rotor are housed, while a slight clearance is maintained at all times between. The rotors and said casing, said screw compressor is arranged so as to supply no oil into the compression space for lubrication, cooling and sealing purposes. The oil-free type screw compressor of this type has been conveniently employed to produce a clean compressed air free of oil mist.
The present invention is not directed to the construction of the oil-free screw compressor itself; however to facilitate an understanding of the present invention, an explanation will be given to the construction of the oil-free screw compressor before describing the present invention.
In an oil-free type screw compressors, no oil is supplied to the compression space in which rotors are housed, but there is the risk of the oil for lubricating the bearings for journalling the rotors entering the compression space. The oil tends to enter the compression space through clearances between the shafts connected to the rotors and the wall of the casing enclosing the shafts. Thus, has been the usual practice to provide oil shield means between the shafts and the compression space.
FIGS. 1 to 6 show a single stage, oil-free screw compressor, as disclosed in U.S. Pat. No. 4,487,563.
A male rotor 1 including a gear thread 1a and shaft portions 12a and 12c and a female rotor 2 including a gear thread 2a and shaft portions 12b and 12d are located in a compression space defined by a casing while being maintained in meshing engagement with each other.
The casing includes a main casing 3, a suction-side casing 4 is connected to the main casing 3 through bolts and an end cover 5 is also connected to the main casing 3 through bolts, with the main casing 3 defining the compression space in the form of two cylindrical bores intersecting each other.
The shaft portions 12a and 12c of the male rotor 1 and the shaft portions 12b and 12d of the female rotor 2 penetrate the casing, and shaft sealing means 8a, 8b, 8c and 8d are mounted on the shaft portions 12a, 12b, 12c and 12d respectively in positions where they penetrate the casing. The shaft sealing means 8a, 8b, 8c and 8d are located near the rotors for sealing the shafts to prevent compressed gas (compressed air) from leaking to outside from the compression space and keep oil for lubricating bearings from entering the compression space from the bearings.
The two rotors 1 and 2 are supported by the casing through radial bearings 6a, 6b, 6c and 6d for bearing radial loads and by thrust bearings 7a, 7b, 7c and 7d for bearing thrust loads.
A pair of meshing timing gears 9 and 10 are mounted on the discharge-side shaft portions of the rotors 1 and 2, to enable the rotors 1 and 2 to rotate while being out of contact with each other. A pinion 11 is mounted on the end of the suction-side shaft portion 12a of the male rotor 1 and driven by a pull gear (not shown). When motive force is transmitted from a drive source to the pinion 11, the male and the female rotors 1 and 2, forming a pair, are rotated in synchronism with each other through the timing gears 9 and 10 while being spaced apart from each other by a small clearance. As a result, gas (air) is drawn by suction through a suction passageway 31 and a suction port (not shown) into a compression chamber defined between the screw threads 1a and 2a of the rotors 1 and 2. As the rotors 1 and 2 rotate, communication between the compression chamber and the suction port is shut off and the compression chamber gradually becomes smaller in volume, so that the gas (air) in the compression chamber is compressed and discharged though a discharge port (not shown) to be used for various purposes. A cooling jacket 32 is formed in the casing 3 so that the jacket 32 surrounds the compression space, and a coolant, such as water, supplied from outside is circulated through the cooling jacket 32.
FIG. 3 shows in detail the shaft portion 12b of the female rotor 2 on its suction side. The rotors 1 and 2 are of substantially the same construction, so that description of the shaft portion 12a of the male rotor 1 on its suction side shall be omitted. A sealing means 13 of a non-contact type seals the shaft portions against gas (air), the non-contact type sealing means comprising rings formed of carbon or resin, with such as tetrafluoroethylene, and arranged in a side-by-side relationship. The radial roller bearing 6b receives a supply of lubricant delivered in jets through an oil feed duct 28 and a port 30 formed in a bearing support 29. After passing the bearing 6b, a major portion of the lubricant is discharged through an oil discharge duct 26. A suction end 24 of the compression space in which the rotors 1 and 2 are located is at a negative pressure at all times and the negative pressure is enhanced during a no-load operation. Thus, air is drawn at all times to a space 55 by suction through an atmosphere communicating duct 16 and a plurality of ports 15 formed in a lantern ring 14, so that the space 55 defined between the lantern ring 14 and the shaft portion 12b is at a negative pressure at all times and the negative pressure becomes enhanced during a no-load operation. Thus, it is necessary to use oil shield means of high sealing ability to prevent the oil discharged from the bearing 6b from being drawn by suction into the compression space. To this end, there is used screw seal means capable of performing a satisfactory sealing function through a pumping action when the shafts are rotated. The screw seal has a sealing ability which is substantially in inverse proportion to the radial clearance between the shaft and the screw seal, so that a screw seal ring 21 of a floating type is used to minimize the radial clearance.
The screw seal ring 21 is formed on its inner surface with a spiral screw thread 23 and a thread root 22 as shown in FIG. 4. As the shaft portion 12b rotates in the direction of an arrow in FIG. 3, the gas (air) in the thread root 22 is pulled due to its viscosity and flows from the space 55 in the direction of the bearing 6b while performing a pumping action to discharge the oil in the direction of the bearing. The screw seal ring 21 is pressed by a corrugated spring or coil spring 20 against the lantern ring 14 into end-to-end contact. Since the seal ring 21 is radially movable, the seal ring 21 can be prevented from coming into contact with the shaft portion 12b if the clearance between the seal ring 21 and the shaft portion 12b is slightly larger than the radial clearance of the bearing 6b, thereby enabling the seal ring 21 to have a high sealing ability. A ring 18 is formed with an oil discharge port 25, and an O-ring 17 is mounted between the lantern ring 14 and the ring 18 to prevent the oil from leaking to an outer periphery of the lantern ring 14.
FIG. 5 shows in detail the shaft portion 12d of the female rotor 2 on its discharge side. The rotors 1 and 2 are of substantially the same construction, so that description of the shaft portion 12c of the male rotor 1 on its discharge side shall be omitted. A sealing means 40 of the non-contact type seals the shaft portions against gas (air), with the sealing means 40 being of the same construction as the sealing means 13. The radial roller bearing 6d and the thrust roller bearing 7b receive a supply of lubricant delivered in jets through an oil feed duct 49 and small ports 51 and 52 formed in an oil feed ring 50. After passing the bearing 6d, a major portion of the lubricant is discharged through an oil discharge duct 48. A discharge end 56 of the compression space is at a positive pressure (higher than the atmospheric pressure) at all times, so that a portion of the gas (air) is released to the atmosphere through the gas sealing means 40 from an atmosphere communicating duct 46. Thus, the pressure in a space 57 is positive at all times, and there is no risk of the oil being drawn by suction into the compression space. However, it is necessary to provide means for preventing the oil from leaking to outside through the atmosphere communicating duct 46 after passing the bearings 6d. The seal ring means for the shaft portions on the discharge side need not have a high sealing ability, so a radially stationary type screw seal ring 42 of a relatively large radial clearance is used. As shown in FIG. 6, the screw seal ring 42 is formed on its inner surface with a spiral screw thread 44 and a thread root 45. As the shaft portion 12d rotates in the direction of an arrow shown in FIG. 5, the seal ring 42 performs a sealing function by its pumping action. An O-ring 53 is mounted on an outer periphery of the screw seal ring 42 to prevent the oil from leaking to outside through the clearance between the casing 3 and the outer peripheral surface of the seal ring 42. The screw seal ring 42 is formed with a plurality of ports 43 for releasing gas (air) to the atmosphere. The shaft sealing means 40 and the screw seal ring 42 are pressed against a snap ring 47 by a corrugated spring 41.
From the foregoing description, it will be appreciated that the oil-free screw compressor is capable of avoiding entry of lubricating oil for the bearings into the compression space, to produce oil-free air under high pressure.
The conventional oil-free screw compressor is constructed as a single stage oil-free screw compressor which is arranged to directly suction air from the atmosphere into the compressor, as shown in Japanese Patent Publication No. Hei 1-44917 or U.S. Pat. No. 4,487,563.
FIG. 7 illustrates a device of the prior art for producing a compressed air by using a single stage type oil-free screw compressor. The oil-free screw compressor 101 suction atmospheric air through its inlet port 101a, compresses the air to a predetermined pressure (usually 7 Kg/cm.sup.2 G, where G indicates a gauge pressure) and discharges the compressed air through its outlet port 101b and feeds it through a discharge pipe 102 to a precooler 103, which acts to cool the compressed air. The compressed air, which cooled by the precooler 103, is fed through a check valve 104 to an aftercooler 105, to further cool the compressed air, is delivered through an outlet pipe 108 to a user side. As shown in FIG. 7, an oil cooler for cooling 106 cools the oil to be fed to the rotor shaft bearings and other portions to be lubricated in the compressor 101; with a coolant cooler 107 being provided for cooling the coolant to be fed into the cooling jacket 32 (FIGS. 1 and 2) of the compressor 101. A cooling fan 109 cools the above-mentioned coolers, and an air outlet opening 110 is provided though which the air from the fan 109 is discharged.
In general, it is required to produce a clean and oil-free compressed air having a pressure in the order of about 7 Kg/cm.sup.2 G. On the other hand, the oil-free screw compressor is required to deal with a considerable increase of temperature due to compression of air, since there is no lubricant oil in the compression space and, consequently, no cooling effect is produced by the lubricant oil.
In the conventional single-stage oil-free screw compressor, the pressure is increased to 7 Kg/cm.sup.2 G by the single stage, so that the temperature of the discharged air is increased to a considerably high temperature such as 320.degree. C.-380.degree. C. Accordingly, it has been required to increase the clearance between the rotors in order to prevent the rotors from contacting with each other due to their thermal expansions, resulting in the total adiabatic efficiency of the compressor being decreased to 50-55%, thereby lowering the performance of the compressor. Furthermore, the temperature of the discharged air is considerably high so that tolerance of the machine is low and material of the machine is limited and, consequently, reliability of the machine is deteriorated.